Modern aircraft includes a hydraulic power system for delivering fluid under pressure to operate such components as rudder, slats, flaps and ailerons. The pump for the hydraulic system is driven from the aircraft engine; so it is essential that there be an auxiliary power system which permits safe operation and landing of the aircraft in the event of engine failure. Such systems are disclosed in U.S. Pat. Nos. 3,660,977, 3,722,217, and 3,800,534, all owned by applicant's assignee. The systems disclosed in those three patents all use a start grain to provide energy to the turbine until turbine speed is high enough to cause the fuel pump to deliver fuel under pressure from the main fuel tank to the decomposition chamber.
Use of a start grain for initiating operation of the system involves an inherent difficulty which derives from the fact that the rate of energy release from the grain may not match the requirements of the system over a wide range of temperatures encountered in use. The rapid burning and rapid energy delivery from a warm grain could result in turbine overspeed which can be damaging to the system. While this need has been met by secondary speed controls in accordance with U.S. Pat. No. 3,800,534, it is still desirable to have a start system which does not require the use of a start grain.
In the event the tank must be located a considerable distance from the fuel pump, it may be desirable to provide a boost pump at the tank outlet to minimize the required tank pressurization and line sizes.